Not Applicable
1. Field of the Invention
This invention pertains generally to signal generation and, more particularly, to a method and apparatus for generation of an arbitrary electronic waveform using photonics.
2. Description of the Background Art
Signal and data generators are well known tools that are commonly used in a number of applications ranging from designing and testing equipment and circuits, to communication and radar systems. A particular type of data generator, referred to as an xe2x80x9carbitrary wavelength generatorxe2x80x9d (AWG), is particularly useful in radar and Electronic Warfare (EW) applications. In addition, it can be used for characterizing circuits and systems by simulating xe2x80x9crealxe2x80x9d physical conditions. In particular, the device can be used for margin testing by simulating amplitude and timing impairments such as cross talk, intersymbol effects, reflections, ground bounce, noise simulations, jitter testing, and the like. High speed AWGs, such as a Tektronix AWG 610, are available that combine waveform generation and editing that enables the user to create a waveform from scratch, automatically transfer a waveform from an oscilloscope, download signals created via computer simulation tools, and modify signals using built-in editors. Real world signal impairments such as jitter, noise, fading, or the like can be easily simulated using such an AWG.
Other types of data generators are available as well. For example, the Agilent Technologies 81200 provides a platform for verifying digital devices under real-world conditions. The device has a high-speed pattern rate, large pattern depth and a scalable configuration that facilitates digital device verification and characterization. Digital-to-analog converters (DACs) and direct digital synthesizers (DDSs) are at the heart of an electronic AWG.
The bandwidth of current technology is limited to a few GHz by the electronic DAC used in AWG systems. What is needed is a new AWG technology that can synthesize waveforms with much large bandwidths and with arbitrary Amplitude Modulation (AM), Frequency Modulation (FM) and Phase Modulation (PM). The present invention satisfies that need, as well as others, as described herein. It can also be used to generate for microwave, millimeter wave and Tera Hertz frequency signals.
The present invention, which will be referred to herein as a photonic arbitrary waveform generator (PAWG), comprises an apparatus that can synthesize waveforms with arbitrary amplitude, frequency, and phase modulation. The invention can be used in place of digital-to-analog converters (DACs) or electronic arbitrary waveform generators (AWGs) to reproduce waveforms with arbitrary amplitude, frequency and phase variations. The invention can also be used to synthesize spectrally pure sine waves over a wide range of frequencies in place of a direct digital synthesizer (DDS).
By way of example, and not of limitation, in accordance with an aspect of the present invention, pulses from a broadband (supercontinuum) optical source are filtered into a plurality of wavelength channels. The intensity of each wavelength channel is adjusted to an appropriate level depending on the desired shape of the envelope of the output electrical waveform. In a wavelength modulation stage, the wavelength channels, which function as samples of the arbitrary output waveform, can also be time differentiated by introducing small incremental time delays between them or the envelope of the sampling wavelength channels can be further stretched, compressed, or inverted in the time domain later by choosing the proper dispersive medium. After proper time domain manipulation, the optical waveform is observed with a combination of high-speed photodetectors and a radio frequency (RF) low-pass filter to produce an output electrical waveform.
By way of further example, and not of limitation, the present invention can have various embodiments.
In a first embodiment, gratings are used for wavelength division, a spatial light modulator (SLM) is used for intensity adjustments, and a dispersive medium is used for time domain treatment. The supercontinuum pulses are passed through the SLM, wherein the attenuation of the individual pixels can be set by adjusting the gray level of that pixel. By knowledge of the properties of the dispersive medium, wavelength dependence of the photodetector, low-pass characteristics of the photodetector, the radio frequency (RF) filter, and the non-uniformity in the intensity of the various wavelength channels, the required attenuation in each channel that would mimic the envelope of the desired waveform can be determined. The delay between the various wavelength channels is minimal, the output optical pulses are passed through the dispersive medium (e.g., a normal single mode fiber, a negative dispersion fiber, etc.) for time domain manipulation. Different spectral components are separated in time domain due to the wavelength dependent group velocity. Either positive or negative dispersion fibers can be used. The negative dispersion will result in a waveform that is the time-reversed image of the waveform produced by positive dispersion. For a given sign of dispersion, the time-reversal can also be achieved by reversing the spectrum modulation using the SLM or any other type of optical filter. The length of the fiber, the main dispersive element in the system, can be adjusted to achieve the desired time domain spread, and hence the electrical bandwidth of the waveform
In a second embodiment, which is a true time delay (TTD) implementation, wavelength division, intensity adjustment of each wavelength channel, and introduction of incremental time delay between wavelength channels is achieved by passing the supercontinuum pulses through a true time delay device. In one embodiment of the true time delay, the device comprises an N input, N output arrayed waveguide grating where all the corresponding inputs and outputs except one set of input-output ports are connected through an incremental time delay. For the PAWG of the present invention, optical attenuators or electo-optic modulators are also incorporated into each delay line. The supercontinuum pulse is then fed to the free input. The corresponding output comprises a series of optical pulses at different wavelengths set by the arrayed waveguide grating and with set incremental time delay between the pulses. The attenuation for each channel is set in the delay stage to appropriate levels. The output from the true time delay device can be further stretched, compressed, or inverted, if necessary, using a second true time delay or using a fiber with appropriate length and dispersion characteristics.
The output from either embodiment is the sampled version of the desired output waveform. This waveform is observed with either a combination of a high-speed photodetector and an RF low-pass filter, or a low-speed photodetector. The bandwidth of the detecting circuit is determined by the time separation between the adjacent sampling wavelength channels, and the amount dispersion (if fiber is used) or time delay (if true time delay is used). The resultant waveform is the desired output waveform.
An object of the invention is to provide synthesize waveforms at considerably higher frequencies than related devices, into the tens of GHz range.
Another object of the invention is to provide the ability to stretch, compress, and even time-invert these waveforms using optical fibers with proper dispersion characteristics.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.